Switching power supply device

ABSTRACT

A primary side resonant circuit is formed by a primary side resonant inductor and a primary side resonant capacitor, and secondary side resonant circuits are formed by secondary side resonant inductors and secondary side resonant capacitors. Equivalent mutual inductances and equivalent mutual capacitances are formed through electromagnetic field resonant coupling between a primary winding and secondary windings, and a multi-resonant circuit including an LC resonant circuit formed in each of the primary side and the secondary side is formed. Electric power is transmitted from the primary side circuit to the secondary side circuit, and resonant energy that is not transmitted from the primary winding and, of energy which the secondary winding has received, resonant energy that is not supplied to an output are each retained in the multi-resonant circuit.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims benefit of priority to Japanese PatentApplication No. 2011-232307 filed on Oct. 21, 2011, and to InternationalPatent Application No. PCT/JP2012/076397 filed on Oct. 12, 2012, theentire content of each of which is incorporated herein by reference.

TECHNICAL FIELD

The present technical field relates to switching power supply devicesand, in particular, to a switching power supply device that transmitselectric power through a multi-resonant circuit.

BACKGROUND

Electronic devices have been reduced in size and weight in recent years,and there has been an increasing market demand for switching powersupplies with increased efficiency and reduced size and weight. Forexample, in the market for flat screen television sets or the like, inwhich output current ripple characteristics are relatively moderate, acurrent resonant half bridge converter in which a sinusoidal resonantcurrent is made to flow in a transformer to cause the transformer tooperate while utilizing an LC resonance phenomenon is being put topractical use while taking advantage of its feature of being highlyefficient.

For example, Japanese Unexamined Patent Application Publication No.9-308243 discloses an LC series resonant DC-DC converter. FIG. 14 is abasic circuit diagram of a switching power supply device described inJapanese Unexamined Patent Application Publication No. 9-308243. Thisswitching power supply device is a current resonant half bridge DC-DCconverter, in which an LC resonant circuit that is formed by an inductorLr and a capacitor Cr and two switching elements Q1 and Q2 are connectedto a primary winding np of a transformer T. A rectifying smoothingcircuit that is formed by diodes D3 and D4 and a capacitor Co is formedon secondary windings ns1 and ns2 of the transformer T.

With the configuration described above, the switching elements Q1 and Q2are complementarily turned on and off with a dead time, and the waveformof a current that flows through the transformer T thus has a sinusoidalresonant waveform. In addition, electric power is transmitted from theprimary side to the secondary side during both on periods and offperiods of the two switching elements Q1 and Q2.

SUMMARY Technical Problem

The switching power supply device described in Japanese UnexaminedPatent Application Publication No. 9-308243, however, suffers from thefollowing issues to be solved.

(1) The LC resonant circuit is formed only at the primary side (or thesecondary side); a mutual inductance Lm is formed equivalently throughmagnetic coupling between the primary winding and the secondary winding;and electric power is transmitted through electromagnetic induction. Aleakage flux that is not involved in the magnetic coupling, however,forms an equivalent leakage inductance, and magnetic energy of asecondary side leakage inductance leads to an electric power loss as aswitching loss of a rectifying diode. In particular, in a case in whichthe magnetic coupling is small, the leakage inductance of the secondarywinding increases, and in turn the electric power loss increases.

(2) In a case in which the magnetic coupling between the primary windingand the secondary winding is small, an impedance increases since aresonant circuit is not formed in the secondary side circuit. Thus,electric power cannot be transmitted efficiently from the primary sideto the secondary side.

(3) Output electric power can be controlled through a change in aswitching frequency fs. For example, the switching frequency fs iscontrolled to be higher when a load is light and to be lower when theload is heavy. The switching frequency fs, however, becomes excessivelyhigh when the load is light or is not present, and the output electricpower goes out of control, leading to such problems as occurrence of anintermittent oscillation operation and a jump in an output voltage.

Accordingly, the present disclosure is directed to providing a switchingpower supply device with increased output stability and increasedelectric power conversion efficiency.

Solution to Problem

A switching power supply device according the present disclosure isconfigured as follows.

(1) The switching power supply device includes a transformer thatincludes at least a primary winding and a secondary winding;

-   -   a primary side circuit that includes a primary side resonant        inductor Lr formed so as to be equivalent to and in series with        the primary winding,    -   at least one primary side resonant capacitor Cr that, together        with the primary side resonant inductor Lr, forms a primary side        resonant circuit, and    -   a primary side alternating current voltage generation circuit        that includes at least two switching elements and that generates        a trapezoidal wave (rectangular wave) alternating current        voltage from an input direct current power supply voltage        through the switching circuits so as to provide the alternating        current voltage to the primary side resonant circuit; and    -   a secondary side circuit that includes a secondary side resonant        inductor Ls formed so as to be equivalent to and in series with        the secondary winding,    -   a secondary side resonant capacitor Cs that forms, together with        the secondary side resonant inductor Ls, a secondary side        resonant circuit, and    -   a secondary side rectifying circuit that includes a rectifying        element and that rectifies an alternating current outputted from        the secondary side resonant circuit so as to obtain a direct        current voltage.

In the switching power supply device, a mutual inductance Lm is formedequivalently through mutual induction between the primary winding andthe secondary winding; a mutual capacitance Cm is formed equivalentlythrough interaction between the primary winding and the secondarywinding; and a multi-resonant circuit that includes a plurality of LCresonant circuits formed at least in each of the primary side circuitand the secondary side circuit is formed.

With the multi-resonant circuit, the primary side resonant circuit andthe secondary side resonant circuit resonate with each other, andelectric power is thus transmitted from the primary side circuit to thesecondary side circuit through electromagnetic field resonant couplingin which currents flow through the mutual inductance Lm and the mutualcapacitance Cm;

-   -   energy that is not transmitted from the primary winding is        retained in the primary side resonant circuit as resonant energy        through a resonance phenomenon;    -   of energy which the secondary winding has received, energy that        is not supplied to an output is retained in the secondary side        resonant circuit as resonant energy through a resonance        phenomenon; and    -   the secondary side resonant circuit forms a current path that is        different from a current path in which the rectifying element is        formed in series to transmit electric power from the primary        winding to the secondary winding.

According to the configuration described above, the equivalent mutualinductance can be formed through the electromagnetic field resonantcoupling between the primary winding and the secondary winding; theprimary side resonant circuit and the secondary side resonant circuitcan be made to resonate with each other through the multi-resonantcircuit; and electric power can be transmitted efficiently from theprimary side circuit to the secondary side circuit through magneticfield resonant coupling. In addition, when the rectifying elementbecomes nonconducting, the inductor Ls and the capacitor Cs resonatewith each other so as to be retained as resonant energy, and thus anelectric power loss can be suppressed.

(2) It is preferable that, when a switching frequency is represented byfs and a resonant frequency at which an input impedance seen by an inputof the multi-resonant circuit connected to the primary side alternatingcurrent voltage generation circuit in a state in which a load isconnected to an output of the secondary side circuit becomes minimal isrepresented by fa, the primary side alternating current voltagegeneration circuit operates within a range in which fa≦fs is satisfiedto control electric power to be transmitted.

According to the configuration described above, irrespective of thestate of the load, the input impedance of the multi-resonant circuitseen by the primary side alternating current voltage generation circuitis an inductive impedance. Thus, a zero voltage switching operation canbe achieved in the switching element that forms the primary sidealternating current voltage generation circuit, and the output electricpower can be controlled to a desired level relative to the change in thestate of the load. In addition, a zero voltage switching (hereinafter,“ZVS”) in a broad sense or zero current switching (hereinafter, “ZCS”)operation in which the waveform of a voltage or a current across twoends of the rectifying element becomes part of a sinusoidal wave so asto lead to a conducting state or a nonconducting state becomes possible,reducing a switching loss. Efficiency can thus be improved.

(3) It is preferable that, as the secondary side resonant circuit isprovided, the switching power supply device operate within a range inwhich fa≦fs≦fc is satisfied to control electric power to be transmittedwhile a predetermined frequency fc at which a resonant operation ismaintained even in a no-load state in which a load is not connected toan output is set.

According to the configuration described above, the output electricpower can be controlled within a desired operation frequency range.

(4) It is preferable that the secondary side resonant capacitor Cs beconnected in parallel with the secondary winding and that the secondaryside rectifying circuit be configured to rectify a voltage of thesecondary side resonant capacitor Cs.

According to the configuration described above, the ZVS or ZCS operationof the rectifying element becomes possible, and thus a switching losscan be reduced, leading to improved efficiency. In addition, using astray capacitance of the winding as the resonant capacitor allows thenumber of components to be reduced to thus reduce the power supplydevice in size and weight.

(5) It is preferable that the secondary side resonant capacitor Cs beconnected in series with the secondary winding and that the secondaryside rectifying circuit be configured to rectify a current in thesecondary side resonant capacitor Cs.

According to the configuration described above, the ZVS or ZCS operationof the rectifying element becomes possible, and thus a switching losscan be reduced, leading to improved efficiency.

(6) It is preferable that the secondary side resonant capacitor Cs beconnected in parallel with the rectifying element that forms thesecondary side rectifying circuit.

According to the configuration described above, the ZVS or ZCS operationof the rectifying element becomes possible, and thus a switching losscan be reduced, leading to improved efficiency. In addition, using astray capacitance of the rectifying element as the resonant capacitorallows the number of components to be reduced to thus reduce the powersupply device in size and weight.

(7) The secondary side rectifying circuit is, for example, a bridgerectifying circuit.

According to the configuration described above, a withstanding voltagerequired for the rectifying element can be reduced.

(8) The secondary side rectifying circuit is, for example, a center taprectifying circuit.

According to the configuration described above, an output current can besupplied through two rectifying elements and two secondary windings, andthus the efficiency can be improved in a use case in which an outputcurrent is large.

(9) The secondary side rectifying circuit is, for example, a voltagedoubling rectifying circuit.

According to the configuration described above, a high voltage can besupplied through a single secondary winding, and thus the efficiency canbe improved in a use case in which an output voltage is high.

(10) It is preferable that the switching element be turned on when avoltage across two ends thereof falls to the vicinity of a zero voltageto carry out a zero voltage switching operation.

According to the configuration described above, a component for theresonant inductor becomes unnecessary, and thus the switching powersupply device can be reduced in size and weight.

(11) It is preferable that the primary side resonant inductor Lr or thesecondary side resonant inductor Ls be a leakage inductance of theprimary winding or the secondary winding.

According to the configuration described above, the number of componentscan be reduced, and the power supply device can thus be reduced in sizeand weight.

(12) It is preferable that a stray capacitance of the primary winding,together with the primary side resonant capacitor Cr, form the primaryside resonant circuit, or that a stray capacitance of the secondarywinding, together with the secondary side resonant capacitor Cs, formthe secondary side resonant circuit.

According to the configuration described above, the number of componentscan be reduced, and the power supply device can thus be reduced in sizeand weight.

(13) It is preferable that a junction capacitance of the rectifyingelement be used as the secondary side resonant capacitor Cs.

According to the configuration described above, the number of componentscan be reduced, and the power supply device can thus be reduced in sizeand weight.

(14) It is preferable that a diode connected in parallel with theswitching element be provided.

(15) It is preferable that the switching element be an FET, the diodeconnected in parallel be a parasitic diode of the FET, and a parasiticcapacitance of the FET be used as a parallel capacitor.

According to the configuration described above, the number of componentscan be reduced, and the power supply device can thus be reduced in sizeand weight.

(16) It is preferable that a resonant frequency of the primary sideresonant circuit be substantially equal to a resonant frequency of thesecondary side resonant circuit.

According to the configuration described above, electric powertransmission efficiency is improved. In addition, a resonant frequencyband of the electromagnetic field resonant coupling can be broadened.

(17) It is preferable that the switching circuit be a circuit in whichfour switching elements are in full bridge connection.

According to the configuration described above, a withstanding voltagerequired for the switching element can be reduced.

(18) It is preferable that the secondary side rectifying circuit be asynchronous rectifying circuit that rectifies in synchronization with anoperation of the switching circuit in the primary side alternatingcurrent voltage generation circuit.

According to the configuration described above, allowing the switchingelement that forms the synchronous rectifying circuit to undergo aswitching operation makes it possible to transmit electric powerreception side energy, which makes it possible to use an electric powerreception side circuit as an electric power transmission circuit. Inthis manner, for example, bidirectional electric power transmissionbecomes possible.

Advantageous Effects of Disclosure

According to the present disclosure, the following advantageous effectsare obtained.

(a) An equivalent mutual inductance Lm is formed through anelectromagnetic field resonant coupling between a primary winding np anda secondary winding ns, and an equivalent mutual capacitance Cm isformed through interaction between the primary winding np and secondarywindings (ns, ns1, ns2). Thus, a primary side resonant circuit and asecondary side resonant circuit resonate with each other through amulti-resonant circuit, and electric power can then be transmittedefficiently from the primary side circuit to the secondary side circuit.

(b) When a rectifying diode becomes nonconducting, a resonant inductorLs and a resonant capacitor Cs resonate with each other so as to beretained as resonant energy, and thus an electric power loss can besuppressed.

(c) Setting an impedance of the multi-resonant circuit as an inductiveimpedance in which the phase of a current is delayed relative to thephase of a voltage at a primary side alternating current voltagegeneration circuit enables ZVS operations of switching elements Q1 andQ2 throughout the entire load range, and the switching loss can thus begreatly reduced to improve efficiency.

(d) ZVS and ZCS operations of a rectifying element become possible, andthe switching loss can thus be reduced to improve efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of a switching power supply device 101according to a first embodiment.

FIG. 2 illustrates a waveform of each element in the switching powersupply device 101 illustrated in FIG. 1.

FIG. 3 is a circuit diagram of a switching power supply device 102according to a second embodiment.

FIG. 4 is a circuit diagram of a switching power supply device 103according to a third embodiment.

FIG. 5 is a circuit diagram of a switching power supply device 104according to a fourth embodiment.

FIG. 6 is a circuit diagram of a switching power supply device 105according to a fifth embodiment.

FIG. 7 is a circuit diagram of a switching power supply device 106according to a sixth embodiment.

FIG. 8 is a circuit diagram of a switching power supply device 107according to a seventh embodiment.

FIG. 9 is a circuit diagram of a switching power supply device 108according to an eighth embodiment.

FIG. 10 is a circuit diagram of a switching power supply device 109according to a ninth embodiment.

FIG. 11 is a circuit diagram of a switching power supply device 110according to a tenth embodiment.

FIG. 12 is a circuit diagram of a switching power supply device 111according to an eleventh embodiment.

FIG. 13 is a circuit diagram of a switching power supply device 112according to a twelfth embodiment.

FIG. 14 is a basic circuit diagram of a switching power supply devicedescribed in Japanese Unexamined Patent Application Publication No.9-308243.

DETAILED DESCRIPTION

First Embodiment

FIG. 1 is a circuit diagram of a switching power supply device 101according to a first embodiment.

The switching power supply device 101 is a circuit that accepts input ofan input power supply Vi through an input unit and stably suppliesdirect current electric power to a load Ro from an output unit. Theswitching power supply device 101 includes each of the followingelements.

-   -   A transformer T that includes a primary winding np and secondary        windings ns1 and ns2.    -   A primary side resonant inductor Lr that is formed so as to be        equivalent to and in series with the primary winding np.    -   At least one primary side resonant capacitor Cr that, together        with the resonant inductor Lr, forms a primary side resonant        circuit.    -   Two switching circuits that are formed, respectively, by        switching elements Q1 and Q2, antiparallel diodes Dds1 and Dds2,        and parallel capacitors Cds1 and Cds2.    -   A primary side alternating current voltage generation circuit        that generates a trapezoidal wave (rectangular wave) alternating        current voltage from an input direct current power supply        voltage through the aforementioned switching circuits and        provides the alternating current voltage to the primary side        resonant circuit.    -   Secondary side resonant inductors Ls1 and Ls2 that are formed so        as to be equivalent to and in series with the secondary windings        ns1 and ns2.    -   Secondary side resonant capacitors Cs1 and Cs2 that, together        with the resonant inductors Ls1 and Ls2, form a secondary side        resonant circuit.    -   A secondary side rectifying circuit that includes diodes Ds1 and        Ds2 and rectifies an alternating current outputted from the        secondary side resonant circuit to obtain a direct current        voltage.    -   A multi-resonant circuit that includes a plurality of LC        resonant circuits provided in the primary side circuit and the        secondary side circuit, in which equivalent mutual inductances        Lm, Lms1 and Lms2 are formed through mutual induction between        the primary winding np and the secondary windings ns1 and ns2        and equivalent mutual capacitances Cm1, Cm2, and Cm3 are formed        through interaction between the primary winding np and the        secondary windings ns1 and ns2

Characteristic configurations of this switching power supply device canbe described briefly as follows.

-   -   An LC resonant circuit is provided at each of the primary side        and the secondary side, and electric power is transmitted        efficiently through electromagnetic field resonant coupling.    -   An LC resonant circuit that is formed by Cs1, Ls1, and Lms1 and        another LC resonant circuit that is formed by Cs2, Ls2, and Lms2        are provided at the secondary side.    -   An LC resonant circuit that is formed by Lr, Cr, and Lm is        provided at the primary side.    -   The multi-resonant circuit is formed by Cr, Lm, Lr, Cs1, Ls1,        Lms1, Cs2, Ls2, Lms2, Cm1, Cm2, and Cm3.

The operations of this switching power supply device 101 are as follows.

(1) The equivalent mutual inductances Lm, Lms1, and Lms2 are formedthrough mutual induction between the primary winding np and thesecondary windings ns1 and ns2; the multi-resonant circuit that isformed by Cr, Lr, Lm, Cs1, Ls1, Lms1, Cs2, Ls2, Lms2, Cm1, Cm2, and Cm3causes the primary side resonant circuit and the secondary side resonantcircuit to resonate with each other; and electric power is transmittedfrom the primary side circuit to the secondary side circuit due to theelectromagnetic field resonant coupling in which resonant currents flowthrough the mutual inductances Lm, Lms1, and Lms2 and currents flowthrough the mutual capacitances Cm1, Cm2, and Cm3.

(2) The inductor Ls1 and the capacitor Cs1 resonate with each other whenthe diode Ds1 becomes nonconducting or conducting.

(3) The inductor Ls2 and the capacitor Cs2 resonate with each other whenthe diode Ds2 becomes nonconducting or conducting.

(4) The switching elements Q1 and Q2 are turned on and off in analternating manner with a dead time, and thus a trapezoidal alternatingcurrent voltage waveform is generated from the direct current voltageVi. This trapezoidal alternating current voltage waveform becomes analternating current waveform of a sinusoidal waveform or of a partiallysinusoidal waveform through a resonance phenomenon by the multi-resonantcircuit that is formed by Cr, Lm, Lr, Cs1, Ls1, Lms1, Cs2, Ls2, Lms2,Cm1, Cm2, and Cm3. Furthermore, the stated alternating current waveformis rectified by the diodes Ds1 and Ds2, and a direct current voltage isthus generated.

(5) When fa represents a resonant frequency at which an input impedanceseen by an input of the multi-resonant circuit that is connected to theprimary side alternating current voltage generation circuit in a statein which a load is connected to an output of the secondary side circuitreaches the minimum, the switching power supply device 101 operateswithin a range in which fa≦fs is satisfied so as to control electricpower transmission.

FIG. 2 illustrates a waveform of each element in the switching powersupply device 101 illustrated in FIG. 1. With reference to FIG. 1 andFIG. 2, the operations of the switching power supply device 101 will bedescribed.

Here, voltages across the gates and the sources of the switchingelements Q1 and Q2 are represented by vgs1 and vgs2; voltages across thedrains and the sources are represented by vds1 and vds2; voltages acrossthe two ends of the diodes Ds1 and Ds2 are represented by vrs1 and vrs2;and a current that flows through a common ground of the secondarywindings ns1 and ns2 is represented by i_(s).

The switching elements Q1 and Q2 are turned on and off in an alternatingmanner with a short dead time, during which the switching elements areboth turned off, and currents that flow through Q1 and Q2 during thedead time are each commutated, leading to the ZVS operation. Theoperation in each period within a single switching cycle is as follows.

(1) State 1 Times t1 to t2

First, the diode Dds1 becomes conducting. Turning on the switchingelement Q1 during a conducting period of the diode Dds1 causes the ZVSoperation to be carried out, and the switching element Q1 becomesconducting. The equivalent mutual inductances Lm, Lms1, and Lms2 areformed through mutual induction between the primary winding np and thesecondary windings ns1 and ns2. In addition, the equivalent mutualcapacitances Cm1, Cm2, and Cm3 are formed through interaction betweenthe primary winding np and the secondary windings ns1 and ns2. In themulti-resonant circuit that is formed by Cr, Lr, Lm, Cs1, Ls1, Lms1,Cs2, Ls2, and Lms2, the primary side resonant circuit and the secondaryside resonant circuit resonate with each other, and electric power istransmitted from the primary side circuit to the secondary side circuitdue to the magnetic field resonant coupling in which resonant currentsflow through the mutual inductances Lm, Lms1, and Lms2. At the primaryside, resonant currents flow through the capacitor Cr and the inductorsLr and Lm. At the secondary side, resonant currents flow through thecapacitor Cs1 and the inductors Ls1 and Lms1 and through the capacitorCs2 and the inductors Ls2 and Lms2. The capacitor Cs1 is charged, andthe capacitor Cs2 discharges. A current is supplied to the load Ro froma capacitor Co. When a voltage vs1 becomes equal to an output voltage voand a voltage vs2 becomes 0 V, the diode Ds1 becomes conducting, leadingto State 2.

(2) State 2 Times t2 to t3

The equivalent mutual inductances Lm and Lms1 are formed through mutualinduction between the primary winding np and the secondary winding ns1.In addition, the equivalent mutual capacitances Cm1, Cm2, and Cm3 areformed through interaction between the primary winding np and thesecondary windings ns1 and ns2. Electric power is transmitted from theprimary side circuit to the secondary side circuit due to theelectromagnetic field coupling. At the primary side, resonant currentsflow through the capacitor Cr and the inductors Lr and Lm. At thesecondary side, resonant currents flow through the inductors Ls1 andLms1, and a current passes through the diode Ds1 to be supplied to theload Ro. Upon the switching element Q1 being turned off, State 3 starts.

(3) State 3 Times t3 to t4

At the primary side, the capacitor Cds1 is charged with a current itthat has flowed through the inductor Lr, and the capacitor Cds2discharges. At the secondary side, the current from the inductor Ls1flows through the diode Ds1 to be supplied to the load Ro. When thevoltage vds1 becomes the voltage Vi and the voltage vds2 becomes 0 V,the diode Dds2 becomes conducting, leading to State 4.

(4) State 4 Times t4 to t5

First, the diode Dds2 becomes conducting. Turning on the switchingelement Q2 during a conducting period of the diode Dds2 causes the ZVSoperation to be carried out, and the switching element Q2 becomesconducting. The equivalent mutual inductances Lm, Lms1, and Lms2 areformed through mutual induction between the primary winding np and thesecondary windings ns1 and ns2. In addition, the equivalent mutualcapacitances Cm1, Cm2, and Cm3 are formed through interaction betweenthe primary winding np and the secondary windings ns1 and ns2. In themulti-resonant circuit that is formed by Cr, Lr, Lm, Cs1, Ls1, Lms1,Cs2, Ls2, Lms2, Cm1, Cm2, and Cm3, the primary side resonant circuit andthe secondary side resonant circuit resonate with each other, andelectric power is transmitted from the primary side circuit to thesecondary side circuit due to the magnetic field resonant coupling inwhich resonant currents flow through the mutual inductances Lm, Lms1,and Lms2. At the primary side, resonant currents flow through thecapacitor Cr and the inductors Lr and Lm. At the secondary side,resonant currents flow through the capacitor Cs1 and the inductors Ls1and Lms1 and through the capacitor Cs2 and the inductors Ls2 and Lms2.The capacitor Cs1 discharges, and the capacitor Cs2 is charged. Acurrent is supplied to the load Ro from the capacitor Co. When thevoltage vs1 becomes 0 V and the voltage vs2 becomes equal to the outputvoltage vo, the diode Ds2 becomes conducting, leading to State 5.

(5) State 5 Times t5 to t6

The equivalent mutual inductances Lm and Lms2 are formed through mutualinduction between the primary winding np and the secondary winding ns2.In addition, the equivalent mutual capacitances Cm1, Cm2, and Cm3 areformed through interaction between the primary winding np and thesecondary windings ns1 and ns2. Electric power is transmitted from theprimary side circuit to the secondary side circuit due to theelectromagnetic field coupling. At the primary side, resonant currentsflow through the capacitor Cr and the inductors Lr and Lm. At thesecondary side, resonant current flows through the inductors Ls2 andLms2, and a current passes through the diode Ds2 to be supplied to theload Ro. Upon the switching element Q2 being turned off, State 6 starts.

(6) State 6 Times t6 to t1

At the primary side, with the current it that has flowed through theinductor Lr, the capacitor Cds1 discharges, and the capacitor Cds2 ischarged. At the secondary side, the current from the inductor Ls2 flowsthrough the diode Ds2 to be supplied to the load Ro. When the voltagevds1 becomes 0 V, and the voltage vds2 becomes the voltage Vi, the diodeDds1 becomes conducting, leading again to State 1.

Thereafter, States 1 through 6 are repeated cyclically.

According to the first embodiment, the following advantageous effectsare obtained.

(a) The equivalent mutual inductances are formed through mutualinduction and the equivalent mutual capacitances are formed throughinteraction between the primary winding np and the secondary windingsns1 and ns2; the primary side resonant circuit and the secondary sideresonant circuit resonate with each other through the multi-resonantcircuit; and electric power can be transmitted efficiently from theprimary side circuit to the secondary side circuit through theelectromagnetic field resonant coupling.

(b) When the diode Ds1 becomes nonconducting, the inductor Ls1 and thecapacitor Cs1 resonate with each other so as to be retained as resonantenergy, and thus an electric power loss can be suppressed.

(c) When the diode Ds2 becomes nonconducting, the inductor Ls2 and thecapacitor Cs2 resonate with each other so as to be retained as resonantenergy, and thus an electric power loss can be suppressed.

(d) Setting the impedance of the multi-resonant circuit to an inductiveimpedance in which the phase of a current is delayed relative to thephase of a voltage at the primary side alternating current voltagegeneration circuit enables the ZVS operations of the switching elementsQ1 and Q2 throughout the entire load range. Consequently, a switchingloss can be greatly reduced to improve efficiency.

(e) The ZVS and ZCS operations of the rectifying diode become possible,and a switching loss can be reduced to improve efficiency.

(f) Providing the LC resonant circuits that are formed by Cs1, Ls1, andLms1, and Cs2, Ls2, and Lms2 at the secondary side makes it possible tocontrol output electric power to a desired output electric power bysetting a frequency fc at which no-load electric power is achieved andby operating the switching power supply device within a range in which aswitching frequency fs satisfies fs≦fc.

(g) Using MOS-FETs for the switching elements Q1 and Q2 makes itpossible to form a switching circuit by using a parasitic capacitanceand an antiparallel diode, and thus the number of components can bereduced to reduce the power supply device in size and weight.

(h) Using leakage inductances of the winding as the resonant inductorsLr, Ls1, and Ls2 makes it possible to reduce the number of components tothus reduce the power supply device in size and weight.

(i) Using mutual inductances of the transformer T as the resonantinductors Lm, Lms1, and Lms2 makes it possible to reduce the number ofcomponents to thus reduce the power supply device in size and weight.

(j) Using stray capacitances of the winding as the resonant capacitorsCs1 and Cs2 makes it possible to reduce the number of components to thusreduce the power supply device in size and weight.

(k) Using junction capacitances of the rectifying diodes Ds1 and Ds2 asresonant capacitors makes it possible to reduce the number of componentsto thus reduce the power supply device in size and weight.

Second Embodiment

FIG. 3 is a circuit diagram of a switching power supply device 102according to a second embodiment. In this example, unlike the switchingpower supply device 101 of the first embodiment, the secondary sideresonant capacitors Cs1 and Cs2 are connected in parallel with thediodes Ds1 and Ds2, respectively. In addition, the antiparallel diodesDds1 and Dds2 illustrated in FIG. 1 are constituted by parasitic diodesof the switching elements Q1 and Q2, respectively.

Operations and effects are similar to those described in the firstembodiment. In particular, in the second embodiment, junctioncapacitances of the diodes Ds1 and Ds2 can be used as the secondary sideresonant capacitors Cs1 and Cs2.

Third Embodiment

FIG. 4 is a circuit diagram of a switching power supply device 103according to a third embodiment. In this example, unlike the switchingpower supply device 101 of the first embodiment, the secondary sideresonant capacitor is constituted by a single capacitor Cs.

Operations and effects are similar to those described in the firstembodiment. In particular, in the third embodiment, the secondary sideresonant capacitor Cs can be constituted by a single capacitor, and thusthe number of components can be reduced.

Fourth Embodiment

FIG. 5 is a circuit diagram of a switching power supply device 104according to a fourth embodiment. In this example, unlike the switchingpower supply device 101 of the first embodiment, leakage inductances ofthe winding are used as the inductors Lr, Ls1, and Ls2, and mutualinductances of the transformer T are used as the inductors Lm, Lms1, andLms2.

Operations and effects are similar to those described in the firstembodiment. In particular, in the fourth embodiment, the number ofcomponents can be reduced.

Fifth Embodiment

FIG. 6 is a circuit diagram of a switching power supply device 105according to a fifth embodiment. In this example, unlike the switchingpower supply device 101 of the first embodiment, the capacitors Cs1 andCs2 are formed so as to be in series with the inductors Ls1 and Ls2,respectively, and capacitors Cs3 and Cs4 are connected in parallel withthe output.

Operations and effects are similar to those described in the firstembodiment. In particular, in the fifth embodiment, the frequency fc canbe set to a desired value by setting the values of the capacitors Cs1 toCs4 as appropriate.

Sixth Embodiment

FIG. 7 is a circuit diagram of a switching power supply device 106according to a sixth embodiment. In this example, unlike the switchingpower supply device 101 of the first embodiment, a bridge rectifyingcircuit is formed as a rectifying circuit.

Operations and effects are similar to those described in the firstembodiment. In particular, in the sixth embodiment, the secondarywinding can be constituted by a single winding, and the capacitor Cs canbe constituted by a single capacitor.

Seventh Embodiment

FIG. 8 is a circuit diagram of a switching power supply device 107according to a seventh embodiment. In this example, unlike the switchingpower supply device 101 of the first embodiment, a bridge rectifyingcircuit is formed as a rectifying circuit, and resonant capacitors Cs1,Cs2, Cs3, and Cs4 are formed in parallel with rectifying diodes Ds1,Ds2, Ds3, and Ds4, respectively.

Operations and effects are similar to those described in the firstembodiment. In particular, in the seventh embodiment, junctioncapacitances of the diodes Ds1, Ds2, Ds3, and Ds4 can be used as theresonant capacitors Cs1, Cs2, Cs3, and Cs4, and thus the number ofcomponents can be reduced.

Eighth Embodiment

FIG. 9 is a circuit diagram of a switching power supply device 108according to an eighth embodiment. In this example, unlike the switchingpower supply device 101 of the first embodiment, the capacitor Cs1 isconnected in series with the inductor Ls, and the capacitor Cs2 isconnected in parallel with the output.

Operations and effects are similar to those described in the firstembodiment. In particular, in the eighth embodiment, the frequency fccan be set to a desired value, and withstanding voltages required forthe rectifying elements (diodes Ds1, Ds2, Ds3, and Ds4) can be reduced.Consequently, a rectifying element with a small conduction loss can beused, which makes it possible to reduce the loss.

Ninth Embodiment

FIG. 10 is a circuit diagram of a switching power supply device 109according to a ninth embodiment. In this example, unlike the switchingpower supply device 101 of the first embodiment, the rectifying circuitis constituted by a voltage doubling rectifying circuit. In addition,the capacitor Cs is connected in series with the inductor Ls, and theresonant capacitors Cs1 and Cs2 are formed so as to be in parallel withthe rectifying diodes Ds1 and Ds2, respectively.

Operations and effects are similar to those described in the firstembodiment. In particular, in the ninth embodiment, a high outputvoltage can be obtained through voltage doubling rectification, andjunction capacitances of the diodes Ds1 and Ds2 can be used as theresonant capacitors Cs1 and Cs2.

Tenth Embodiment

FIG. 11 is a circuit diagram of a switching power supply device 110according to a tenth embodiment. In this example, unlike the switchingpower supply device 101 of the first embodiment, the resonant capacitorsCr and Cs, the mutual inductances (Lm, Lms), the primary winding np, andthe secondary winding ns are formed at locations that are different fromthose of the ninth embodiment. In addition, leakage inductances of thewinding are used as the inductors Lr and Ls, and the secondary siderectifying circuit is constituted by a synchronous rectifying circuit.

Operations and effects are similar to those described in the firstembodiment. In particular, with the tenth embodiment, the followingadvantageous effects are obtained.

-   -   By arranging the resonant capacitors Cr and Cs, the primary        winding np, and the secondary winding ns as appropriate, the        area to be occupied by the mounted components can be utilized        efficiently.    -   Setting the resonant capacitor Cr to a ground potential of the        input side makes it easier to detect a voltage or a current of        the resonant capacitor Cr, and controlling the switching element        by detecting this voltage makes it possible to control the        electric power. To detect the current, a capacitor with a small        capacitance can be connected in parallel with the resonant        capacitor Cr, and by detecting a current that flows through this        capacitor with a small capacitance, the current that flows        through the resonant capacitor Cr can be detected on the basis        of a ratio corresponding to the capacitance ratio.    -   Using the leakage inductances of the transformer of the winding        as the resonant inductors Lr and Ls makes it possible to reduce        the number of components to thus reduce the power supply device        in size and weight. In addition, using the mutual inductances        between the windings as the resonant inductors (Lm, Lms) makes        it possible to reduce the number of components to thus reduce        the power supply device in size and weight.    -   Constituting the secondary side rectifying circuit by a        synchronous rectifying circuit makes it possible to reduce a        rectification loss. In addition, causing the switching element        that forms the synchronous rectifying circuit to undergo a        switching operation makes it possible to transmit electric power        reception side energy, which in turn makes it possible to use an        electric power reception side circuit as an electric power        transmission circuit. In this manner, for example, bidirectional        electric power transmission becomes possible.        Eleventh Embodiment

FIG. 12 is a circuit diagram of a switching power supply device 111according to an eleventh embodiment. This example differs from theswitching power supply device 101 of the first embodiment in thefollowing respects.

-   -   The leakage inductance of the primary winding np is used as the        inductor Lr; the mutual inductance is used as the inductor Lm;        and the winding capacitance is used as the capacitor Cr1. The        leakage inductance of the secondary winding ns is used as the        inductor Ls; the mutual inductance is used as the inductor Lms1,        and the winding capacitance is used as the capacitor Cs1.    -   The primary side alternating current voltage generation circuit        is constituted by a full bridge circuit.    -   Self-resonant coils are used for the primary winding np and the        secondary winding ns.    -   Instead of a magnetic core, an air core is used to couple the        primary winding np and the secondary winding ns.    -   The rectifying circuit at the secondary side is constituted by a        synchronous rectification bridge rectifying circuit.

Operations and effects are similar to those described in the firstembodiment. In particular, in the eleventh embodiment, using theself-resonant coils for the primary winding and the secondary windingmakes it possible to constitute the electric power transmission systemin a simple manner. In addition, constituting the primary sidealternating current voltage generation circuit or the rectifying circuitby a full bridge circuit allows the withstanding voltage required forthe switching element to be reduced, which makes it possible to use aswitching element with a small conduction loss. Consequently, aswitching element with a small conduction loss can be used, which makesit possible to reduce the loss.

Twelfth Embodiment

FIG. 13 is a circuit diagram of a switching power supply device 112according to a twelfth embodiment. In this example, unlike the switchingpower supply device 101 of the first embodiment, an equivalent mutualcapacitance Cm4, in addition to the equivalent mutual capacitances Cm1,Cm2, and Cm3, is formed through interaction between the primary windingnp and the secondary windings ns1 and ns2.

In this manner, in a case in which there are multiple secondarywindings, four or more equivalent mutual capacitances are generatedthrough interaction between the primary winding and the secondarywindings, and the LC resonant circuits at the primary side and thesecondary side may be made to undergo electromagnetic field resonantcoupling by using these equivalent mutual capacitances Cm1, Cm2, Cm3,and Cm4.

The invention claimed is:
 1. A switching power supply device,comprising: a transformer including at least a primary winding and asecondary winding; a primary side circuit including a primary sideresonant inductor formed so as to be equivalent to and in series withthe primary winding, a primary side resonant capacitor forming, togetherwith the primary side resonant inductor, a primary side resonantcircuit, and a primary side alternating current voltage generationcircuit including at least two switching elements and configured togenerate a trapezoidal wave alternating current voltage from an inputdirect current power supply voltage and to provide the trapezoidal wavealternating current voltage to the primary side resonant circuit; and asecondary side circuit including a secondary side resonant inductorformed so as to be equivalent to and in series with the secondarywinding, a secondary side resonant capacitor forming, together with thesecondary side resonant inductor, a secondary side resonant circuit, anda secondary side rectifying circuit including a rectifying element andconfigured to rectify an alternating current outputted from thesecondary side resonant circuit to obtain a direct current voltage,wherein a mutual inductance is formed equivalently through mutualinduction between the primary winding and the secondary winding; amutual capacitance is formed equivalently through interaction betweenthe primary winding and the secondary winding; and a multi-resonantcircuit is formed that includes a plurality of LC resonant circuitsformed at least in each of the primary side circuit and the secondaryside circuit, and wherein, with the multi-resonant circuit, the primaryside resonant circuit and the secondary side resonant circuit resonatewith each other, and electric power is transmitted from the primary sidecircuit to the secondary side circuit through an electromagnetic fieldresonant coupling in which currents flow in the mutual inductance andthe mutual capacitance; energy that is not transmitted from the primarywinding is retained in the primary side resonant circuit as resonantenergy through a resonance phenomenon; of energy which the secondarywinding has received, energy that is not supplied to an output isretained in the secondary side resonant circuit as resonant energythrough a resonance phenomenon; and the secondary side resonant circuitforms a current path that is different from a current path in which therectifying element is formed in series to transmit electric power fromthe primary winding to the secondary winding.
 2. The switching powersupply device according to claim 1, wherein, when a switching frequencyis represented by fs and a resonant frequency at which an inputimpedance seen by an input of the multi-resonant circuit connected tothe primary side alternating current voltage generation circuit in astate in which a load is connected to an output of the secondary sidecircuit is represented by fa, the primary side alternating currentvoltage generation circuit operates within a range in which fa≦fs issatisfied to control electric power to be transmitted.
 3. The switchingpower supply device according to claim 2, wherein, as the secondary sideresonant circuit is provided, a predetermined frequency fc at which theswitching power supply device operates while maintaining a resonantoperation even in a no-load state in which a load is not connected to anoutput is set, and the switching power supply device operates within arange in which fa≦fs≦fc is satisfied to control electric power to betransmitted.
 4. The switching power supply device according to claim 1,wherein the secondary side resonant capacitor is connected in parallelwith the secondary winding, and the secondary side rectifying circuitrectifies a voltage at the secondary side resonant capacitor.
 5. Theswitching power supply device according to claim 1, wherein thesecondary side resonant capacitor is connected in series with thesecondary winding, and the secondary side rectifying circuit rectifies acurrent in the secondary side resonant capacitor.
 6. The switching powersupply device according to claim 1, wherein the secondary side resonantcapacitor is connected in parallel with the rectifying element thatforms the secondary side rectifying circuit.
 7. The switching powersupply device according to claim 1, wherein the secondary siderectifying circuit is a bridge rectifying circuit.
 8. The switchingpower supply device according to claim 1, wherein the secondary siderectifying circuit is a center tap rectifying circuit.
 9. The switchingpower supply device according to claim 1, wherein the secondary siderectifying circuit is a voltage doubling rectifying circuit.
 10. Theswitching power supply device according to claim 1, wherein theswitching element is turned on when a voltage across two ends thereoffalls to a zero voltage to carry out a zero voltage switching operation.11. The switching power supply device according to claim 1, wherein theprimary side resonant inductor or the secondary side resonant inductoris a leakage inductance of the primary winding or the secondary winding.12. The switching power supply device according to claim 1, wherein astray capacitance of the primary winding, together with the primary sideresonant capacitor, forms the primary side resonant circuit, or a straycapacitance of the secondary winding, together with the secondary sideresonant capacitor, forms the secondary side resonant circuit.
 13. Theswitching power supply device according to claim 1, wherein a junctioncapacitance of the rectifying element is used as the secondary sideresonant capacitor.
 14. The switching power supply device according toclaim 1, wherein a diode connected in parallel with the switchingelements is provided.
 15. The switching power supply device according toclaim 14, wherein the switching element is an FET; the diode connectedin parallel is a parasitic diode of the FET; and a parasitic capacitanceof the FET is used as a parallel capacitor.
 16. The switching powersupply device according to claim 1, wherein a resonant frequency of theprimary side resonant circuit is substantially equal to a resonantfrequency of the secondary side resonant circuit.
 17. The switchingpower supply device according to claim 1, wherein the primary sidealternating current voltage generation circuit is a circuit in whichfour switching elements are in full bridge connection.
 18. The switchingpower supply device according to claim 1, wherein the secondary siderectifying circuit is a synchronous rectifying circuit that rectifies insynchronization with a switching operation of the primary sidealternating current voltage generation circuit.